Sheathing of electric cables



Oct. 9. 1956 R. M. FAIRFIELD ETAL 2,765,527

SHEATHING OF ELECTRIC CABLES 3 Sheets-Sheet 1 Filed July 8 1949 Oct. 9,1956 R. M. FAIRFIELD ETAL 2,765,527

SHEATHING OF ELECTRIC CABLES 5 Sheets-Sheet 2 Filed July 8, 1949Inventors PM 8/0 Mel eoo Fa/k'fl'elq Oct. 9, 1956 R. M. FAIRFIELD ET AL2,765,537

- SHEATHING OF ELECTRIC CABLES Filed July 8, 1949. 3 Sheets-Sheet 3 5o15 47b 7 r v Patented Oct. 9, 1956 ice STEATG F ELECTRIC CABLES RonaldMcLeod Fairtield, Brarnshiii, near Eversley, Denis Wiliiam Aldridge,Rainhili, and William Travis, Southport, England; said Fairiield andsaid Aldridge assignors to British Insulated Qalienders Cabies Linnited, London, England, and said Travis assignor to Connollys (Blackley)Limited, Manchester, Engiand, both British companies Application July 8,1949, Serial No. 103,722 Claims priority, application Great Britain July26, 1948 12 Claims. (Cl. 29-5 17) The invention relates to the step inthe manufacture of electric cables in which a seamless metal sheath isapplied over the cable body, which may consist of a single insulatedcore or a group of cores with or without other members. The invention isconcerned with the application of a sheath of metal of a substantiallyhigher extrusion temperature than the lead; particularly it deals withaluminium. The method of application of the sheath employed is that inwhich there is extruded over the cable body an oversize sheath, that is,a sheath with an internal diameter greater than the external diameter ofthat body, which is then moved forward from the press, cooled in transitand operated upon to reduce it to a size appropriate for the particularcable body. Where a close fitting sheath is required, the diiierencebetween the initial internal diameter of the sheath and the externaldiameter of the cable body is limited to the reduction in internaldiameter that can be obtained without producing undesirableworkhardening of the sheath, for annealing obviously presentsdifficulties. Where a small residual clearance is required the initialclearance will be correspondingly greater. However, in both cases thepermissible initial clearance Will usually be small and the problem isencountered of avoiding damage to the cable body by heat to which it issubjected, most intensely when in the neighbourhood of the dies of thepress.

The risk of damage to the cable body by heat is reduced by maintaining aclearance between the cable body and the sheath until the sheath hasreached a safe temperature, by which We mean a temperature at or belowwhich the sheath will not during its further cooling transmit heat tothe cable body under the conditions of contact prevailing between thesheath and the cable body at such a rate as to have any substantialadverse eiiect upon the cable body. The risk may be still furtherreduced by evacuating the clearance.

The type of press at present used for the aluminium sheathing of cablesis that in which the metal is inserted in the container in the form ofhot solid billets accurately shaped to fit the container. Through thecentre of the container passes the cable, separated from the hot metalby a suitably cooled tubular shield and guide. In this form of pressextrusion generally takes place in the direction of the stroke of theram in the container. The extrusion cycle comprises a short chargingperiod during which the billets are inserted in the container round thecentral tubular shield and an extrusion stage during which a length ofsheath is formed which moves forward with the cable body.

Where the operation of extruding a sheath on a length of cable bodycannot proceed without interruption, as where, in the case of a billetpress, the charge is insutficient to sheath more than a fraction of thelength of the cable body, there is, despite the presence of a clearancebetween the cable body and the sheath, risk of the cable body becominglocally overheated during the charging period, when the sheath isstationary in the press. By the present invention we reduce the risk ofsuch local overheating of the cable body by operating oversize sheathduring the interval between two successive periods of extrusion toeffect a reduction in its cross-sectional area and reduce its internaldiameter and elongate it. This will provide for relative movement at theextrusion die between the cable body and its sheath during the periodwhen the sheath is stationary in the press. Accordingly our inventionincludes a method of applying a sheath of aluminium or other metalhaving a substantially higher extrusion temperature than lead, whichcomprises extruding an oversize seamless sheath of such metal on to thecable body by a discontinuous extrusion operation and after cooling thesheath to a safe temperature, operating upon the sheath to effect areduction in its cross-sectional area and reduce its internal diameterand elongate it, during the interval between two successive periods ofextrusion. This reducing operation may in some cases be confined to theintervals between successive periods of extrusion. It may take placethroughout the interval or during some part or parts of the interval.

The invention will now be more fully described, frequent reference beingmade to the accompanying drawings which are all diagrammatic and not toscale. In these drawings Figure l is an elevation illustrating one formof arrangement for carrying out the aluminium sheathing of insulatedelectric cables,

Figure 2 is a greatly enlarged cross-section on the line llII of Figure1, showing an example of a sheathed cable as it leaves the extrusionpress shown in Figure 1,

Figure 3 is a greatly enlarged cross-section on the line Ill-III ofFigure l of the same cable after leaving the drawing die of Figure 1 bywhich its sheath has been reduced to a size appropriate for the cablebody,

Figure 4 is a plan showing a modified form of the arrangement of Figure1,

Figure 5 is an elevation illustrating a second form of arrangement forcarrying out the aluminium sheathing of insulated electric cable,

Figure 6 is an enlarged view of the drawing die carriage and a supportroller carriage of Figure 5,

Figure 7 is a cross-sectional view on the line VIIVII of Figure 6 of thedie carriage and its running track shown in Figure 6,

Figure 8 is an elevation of a modified form of arrangement employing amovable drawing die,

Figure 9 is a fragmental elevation on a larger scale of the arrangementshown in Figure 8, and

Figure 10 is a fragmental view, partly in section and partly inelevation, showing an alternative form of cooling device.

In the method illustrated in Figure l, the cable body 1 is drawn oii asupply drum 2 and fed into the press 3, being passed axially through thehydraulic press cylinder 4 and the extrusion ram 5. it is led throughthe con tainer 6 of the extrusion press by a tubular guide 7. Thecontainer is charged with tubular billets made in halves placed togetherto enclose the cable body and shaped to fit the annular space betweenthe Wall of the container and the guide '7. During the extrusion part ofthe cycle, the ram 5 moves forward in the container 6 and a length ofsheath 8 is formed which surrounds the cable body 1 with a clearance 9,as shown for example in Figure 2. This clearance contains air or othergaseous fluid. The gaseous fiuid may be maintained under a pressure inexcess of ambient atmospheric pressure though care must be taken thatthis is not high enough to distend the hot sheath at the die.Alternatively, save in the case of cables having an impregnated fibrousdielectric, it may be maintained at less than atmospheric pressure. Toprovide for the application of pressure or vacuum, the guide tube 7 mayextend rearwards, through the ram 5 and the hydraulic cylinder 4, andterminate in an appropriate sealing gland. The clearance between thecable body and the extruded sheath can then be maintained under highvacuum by connecting the rear end of the tube 7 to a suitable vacuumplant or under pressure by connecting it to a source of compressed airor gas. By maintaining the gaseous fluid at a very low pressure, inother words, by maintaining a high vacuum in the clearance, thetransmission of heat can be reduced to a very low value, for theconditions are then comparable to those which obtain in a vacuum flaskbecause the internal surface of the extruded sheath will have a highlypolished silvery surface, and consequently very low rate of heatemission.

On leaving the press, the sheath 8 passes over a number of rollers 10which maintain it in axial alignment with the dies of the press for ashort distance. Whilst so supported it is cooled by water sprays from aline of jets 11 in a header 12, the water being collected in a tank 13,from which it may be passed to a cooler and pumped back to the jets.Alternatively, with the object of cooling the sheath as soon as possibleafter its formation, it may pass from the extrusion die into and througha liquid-filled cooling tube of the form shown in Figure 10. One end ofthis tube 60 is coupled to the die or its holder 61 and the other isfurnished with a sealing gland 62 through which the sheathed cable 8leaves the tube. Provision may be made to vary the rate of flow ofcooling liquid through the tube or to adjust its temperature or to doboth.

After leaving the cooling arrangement the sheath 8 and its contents areguided by rollers 14 into a drawing die 15 through which it is pulled byany suitable form of haul-off device, for example, a power drivencapstan 16 round which the sheathed cable makes at least one completeturn before passing to a take-up drum 17. The drawing die 15 reduces thediameter of the tubular sheath 8 to an appropriate extent, for exampleso that the sheath fits the cable body, as shown in Figure 3.

The reduction in sheath diameter brought about by the die is effectedwithout very materially changing the wall thickness of the sheath. Itwill be apparent therefore that the drawing down of the sheath 8 by thedie 15 is also accompanied by an elongation of the sheath. It followsthat within the sheath on the entry side of the die 15, throughout thelength between the press 3 and this die 15, the cable body 1 is movingforward relative to the sheath 8 at a fraction of the speed of the cableon the exit side of the drawing die, the proportion being determined bythe ratio of reduction of cross-sectional area of sheath in the die. Theheating effect on the body of the cable whilst the sheath is stationaryin the press during the recharging of the container is reduced byarranging for relative movement between the cable body 1 and the part ofthe sheath 8 between the press 3 and the drawing die 15 to take placeduring the charging period. Preferably the normal relative movement,that is the relative speed of cable body to sheath during the extrusionpart of the cycle, is maintained, or even increased during the chargingperiod.

This result can be attained by making the speed of extrusion of thesheath 8 from the press 3 greater than the speed of entry of the sheath8 into the reducing die 15 so that with the aid of guide rollers 19 adependent loop 18 of slack cable is formed during the extrusion part ofthe cycle which can be then taken up by continuation of the drawingthrough the charging period. If the drawing continues at the same speedthroughout the cycle the relative speed of the cable body to the sheathis constant. During the charging period the speed of the sheath at thepress is zero and hence the speed of the cable body through the pressfalls to the relative speed of cable body to sheath and may be found tobe small. The value of this relative speed of the cable body to thesheath and the minimum length of oversize sheath required, under theseconditions, in the loop 18 at the end of the extrusion period canreadily be calculated from a knowledge of the increase in length broughtabout by the drawing die, the duration of the extrusion period, and thelinear speed of extrusion of the sheath. For example, if the speed ofextrusion is 2.5 feet/sec, the elongation 9%, the extrusion period 33seconds and the charging period separating two successive extrusionperiods 5 seconds:

The length of oversize sheath extruded/cycle =2.5 33=82.5 feet. Thelength of elongated sheath passing over capstart/cycle =82.5X1.09= feet.Speed of take-up of elongated sheath throughout 90 the cycle =237feet/sec.

The length of elongated sheath taken up during the charging period=5X2.37l1.85 feet.

If the relative speed of the cable body to sheath is found to be too lowto avoid risk of overheating of the body during the charging period, itcan be increased by speeding up the rate of drawing during that part ofthe cycle. This would involve increasing the accumulation of slack inthe loop 18 during the extrusion period. In other words, to maintain thesame average rate of output of sheathed cable, the speed of drawingwould be reduced during extrusion and increased during charging.

Where, to obtain a suitable relative speed of the cable body to sheath,it is necessary to form a large loop, it may be advisable to takeprecautions to prevent the sheath from being stretched at the die of thepress by the weight of sheathed cable in the loop. This may be done inone or more of several ways: the pay-out reel 2 may be appropriatelybraked during the charging period to place the cable body under tensionor the extruded sheath may be passed between a pair of grooved rolls 20which are braked for the duration of the charging period, or the centralpart of the loop may be supported on rollers 21 on a vertically slidableframe 22 supported by a counterweight 23 which partially balances theweight of the loop, or the sheathed cable may be guided by rollers 190,as shown in Figure 4, to form a loop 18a in a horizontal orappropriately inclined plane on an appropriate support or supports. Thelatter may take the form of a smooth table 24 on which the sheath willreadily skid. A series of long rollers 25 lying with their axes parallelto the axis of extrusion may be incorporated in the table. In caseswhere the sheathed body is of large diameter it may be desirable toprovide some positively acting means for causing the sheathed cable bodyto assume a loop of the form required. For example, additional rollersmay be provided on the frame 22 on diametrically opposite sides of thesheath to the rollers 21 and a positive drive may be applied to theframe during an initial part or during the whole of its downwardmovement.

Instead of arranging for the formation of a loop of slack during ttheextrusion part of the cycle, the drawing die can be supported on acarriage and arranged to be moved from the capstan or other haul-offdevice towards the press during the charging part of the cycle and movedaway from the press during the extrusion part of the cycle. This willhave the effect of reducing the speed of drawing during the extrusionpart of the cycle, whilst during the charging part the speed of thecable body through the press will be a constant fraction of the speed oftravel of the drawing die. As the charging period will desirably beshort compared with the extrusion period, the speed of approach of thedie towards the press may be relatively high, rendering possible a speedof drawing during the charging period sufficiently high to cause thecable body to be moved through the press at a speed that is adequate toprevent overheating of the cable body. Disregarding thermal contractionelfects, this speed may be readily calculated as follows:

Let the speed of extrusion=S, and the speed of travel of the drawing dieduring the extrusion part of the cycle be Ve and during the chargingpart of the cycle Vc (negative), then if K is the ratio of elongatedlength of sheath to initial length of sheath, we have,

Increase in speed o travel of sheath due to elongation during extrusionperiod=K (SVe)(SVe) Linear speed of sheath at capstan during extrusionperiod =S+K(SVe)(S-Ve) and Linear speed of sheath at capstan duringcharging of press =-Vc(K-1) Assuming that the die moves forward duringthe whole of the extrusion period and back towards the press during thewhole of the charging period we have where te and to are the durationsof extrusion and charging periods, respectively. Hence effectiveperipheral speed of capstan during charging =Ve- K 1 2 which is thespeed of travel of the core through the press during the chargingoperation. It will generally be impracticable without arranging for theformation of a loop during the extrusion period, to draw down the sheathat such a rate during the charging period as will enable the capstanspeed to be maintained constant throughout the cycle. For constantcapstan speed Generally the permissible value of K (which is restrictedby the need to avoid excessive work hardening of the sheath) and thevalue of the ratio of to te+ to will be such as to render Ve greaterthan S. If during the extrusion period the drawing die travels fasterthan the extruded sheath it will move away from the step between thedrawn down and undrawn parts of the sheath and over a reduced part ofthe sheath so that during an initial part of its return journey nodrawing action will take place. Consequently unless there is a loop ofcable for the capstan to draw upon during this initial part of thecharging period or a complete stoppage of the oap stan is provided for,rupture of the sheath will occur at the press, where its strength islowest.

It will be apparent from Equation (2),

capstan speed during charging=Ve- (K 1) that the speed of travel of thecore through the press during charging, will be a maximum when Ve hasits maximum practical value, namely, when during theextrusion part ofthe cycle the drawing die moves forward at the speed of extrusion, thatis, when the operation of drawing down the sheath is confined to thecharging period. If, for example, the length of the extrusion period is33 seconds and tht of the charging period 5 seconds, and the permissibleelongation of the sheath is the speed of travel of the cable bodythrough the press will be found, by substituting in Equation. (2), to be.668. or, where S is, say, 2.5 feet/sec, 1.65 feet/sec. However, toobtain this maximum speed of travel of the cable body through the pressduring charging, the length of particular example quoted it will be 82.5feet. Since V0 Va to the rate of travel of the drawing die during thecharging operation will also be high, namely 16.5 feet/sec.

The length of travel of the drawing die and its speed may be reduced byfurther reducing the effective speed of the capstan during the chargingperiod as compared with its speed during the extrusion period, so as toprovide for a movement of the cable body through the press during thecharging period that will sufiice to avoid risk of local over-heating ofthe cable body during such period. For instance, we may arrange for themovement of the cable body to be not less than 0.2 feet/sec. during thecharging period. The speeds and length of travel of the die to providesuch minimum speed of cable body movement can then be found fromEquation (2), from which, for the example of press quoted above, Ve=0.30feet/sec, Vc=2.0 feet/sec. and the length of travel of the die=l0 feetWhere it is desired to provide for a movement of the cable body throughthe press during the charging period equal to some particularproportion, say

n. 7L of the speed of travel of the body during the extrusion period,the speeds and length of travel of the die to effect this can be foundfrom the equation from which (for the example of press quoted above,

a number of rollers 33 each mounted on a separate wheeled carriage 34which also runs on the track 32. The die carriage 31 and the supportroll carriages 34 are coupled together by long flexible couplings 35 toform a train, so that movement of the die carriage towards the presswill cause the rollers 33 between it and the capstan 16 to separate andthe rollers 33 between it and the press 3 to approach one another, andmovement in the opposite direction will have a converse effect. Thecarriages may be fitted with rubber or spring buffers 36 and theflexible couplings are preferably resiliently extensible to reduce shockat the drawing die, which with the same object in view may beresiliently mounted in its support 37. The number of support rollcarriages 34 required will naturally depend upon the size of cable beingproduced and upon the length of travel of the drawing die. For example,if the length of travel is 10 feet (which quired to ensure a speed oftravel of the cable body through the example of press quoted above ofnot less than 0.2 feet/sec. during the charging operation) it may befound that, in the case of a cable of one inch external diameter asingle support roll carriage on each side of the die carriage willsufiice.

it will be understood that with the arrangement shown in Figure 5, thespeed of the capstan must be very closely controlled and closelyco-ordinated with the speed of travel of the drawing die both during theextrusion period and during the charging period to avoid imposingexcessive tension on the hot sheath in the neighbourhood of the press.To this end we may drive the die carriage 31 by a shunt wound D. C.motor 38 driving, through reduction is the length regearing 39, a pinion40 which engages a rack 41 extending alongside one rail of the track.The motor and gearing are preferably mounted as low as possible toreduce the overall height both of the die carriage and of all thesupport roll carriages. The capstan may also be driven by a shunt woundD. C. motor. The speed of this motor and that of the carriage motor 38are accurately adjusted to a given ratio during the extrusion stroke andto another given ratio during the charging stroke, the die carriagemotor being reversed for the latter. This changeover of relative speedmay be brought about at the beginning of the extrusion period by a limitswitch actuated by the press ram at the beginning of its working strokeand again by a second limit switch actuated by the ram at the end of itsworking stroke. Alternatively, or in addition, a monitoring control maybe imposed upon the capstan drive dependent upon the measured tension inthe sheathed cable. This may be done by allowing the formation of aslight loop between the capstan and the nearest support roll and varyingthe speed of the capstan motor in accordance with the depth of the loop.The power supply to the motor may be by way of a third rail and aslipper on the carriage or by cable 42 payed out from a drum 43 at oneend of the track. As an additional precaution the sheath may be clampedduring the charging period as shown at 20 in Figure 1.

Where the drawing down operation is confined to the charging period, theneed to provide for the close co-ordination of the speed of the carriagewith that of the capstan both during extrusion and during chargingperiods to avoid excessive tension in the hot sheath may be eliminatedby driving the capstan through a slipping coupling during the extrusionperiod and during the charging period applying a positive drive that isaccurately co-ordinated with the speed of travel of the drawing dietowards the press. An example of such an arrangement is showndiagrammatically in Figures 8 and -9. A drawing die is mounted betweenthe adjacent ends of two axially aligned tubular rams 46a, 46b ofopposed twin hydraulic cylinders 47a, 47b axially aligned with theextrusion axis of the press 3 and disposed between the press and adraw-off capstan 16. Pressure fluid is admitted to both cylinders tomaintain the rams in tension, movement of the rams and the drawing diebeing effected by creating a pressure difference between the fluid inone cylinder and that in the other. The capstan 16 is driven through aslipping coupling and serves during the extrusion part of the cyclemerely to overcome friction between the sheathed cable and the walls ofthe tubular fixed cable guides 48a, 48b extending through the hydrauliccylinders 47a, 47b and those of the rams 46a, 46b. During the returnmovement of the drawing die a separate, positive drive is applied to thecapstan 16 by means of twin chains 49 attached to the die mounting block50. These chains pass over twin sprockets 51 which when the chains arebeing drawn towards the press, ratchet-drive a pinion 52 which engages aspur wheel 53 which through a pinion 54 and spur wheel 55 drives apinion 56 which engages a ring of teeth 57 on one side of the capstan16. The diameters of the sprockets, pinions and spurs and toothed ringare chosen to impart an effective peripheral speed to the capstan 16equal to (K-1) x (the speed of travel of the die), where K is the ratioof elongated length to initial length of sheath, as before. During thereturn movement of the rams the direction of rotation of the sprocketsis reversed but due to the incorporation of the ratchet drive no reversemovement is applied to the capstan.

It will be appreciated that in the arrangement described in thepreceding paragraph, the stroke of the reciprocating die equals thelength of sheath extruded per cyclein the example of press quotedearlier in this specification, a matter of 82.5 feet. The apparatus isthus very long. This disadvantage may be reduced by using a telescopicram in place of each of those shown in Figure 8 or by dispensingwith oneof the twin cylinders in which case it would be necessary to providesupporting carriages for the remaining ram and probably for the cablealso. For example, the die carriage described with reference to Figures4, 5 and 6, could be driven by a hydraulic ram instead of an electricmotor and, if desired, the drive to the capstan could be taken from thisram, provision being made to change the transmission ratio at the end ofeach stroke.

Naturally we may use the double hydraulic ram arrangement shown inFigure 8 to provide for reciprocation of the drawing die, and employ aseparate but co-ordinated two-speed drive to the draw-off capstan or itsequivalent. In this case, we can, as will be clear from the descriptionof the arrangement shown in Figures 5, 6 and 7 greatly reduce the lengthof the apparatus by providing for a drawing down action both duringextrusion and charging parts of the cycle. Thus where it is satisfactoryto provide for a speed of travel of the cable body through the press of0.2 feet/sec. during the charging operation, the stroke of thereciprocating die may be reduced to 10 feet, for the aforesaid exampleof press.

It is to be understood that although we have described and shownexamples of apparatus in which the required reduction is effected in asingle drawing die, the invention is not limited in this respect andthat two or more drawing dies may be used in tandem, each die effectingan appropriate proportion of the required reduction in crosssectionalarea of the sheath. It is also to be understood that, although thediagrammatic drawings show the drawing die as a fixed wall type of die,the term drawing die used herein is not to be construed as limited tosuch a die but as including an arrangement of rolls (some or all ofwhich may be power driven) of which the peripheral surfaces co-operateto form a die adapted to reduce and elongate a sheath of appropriateexternal dimensions as the sheath is passed through the die or the diemoved over the sheath.

What we claim as our invention is:

1. A method of applying a sheath of metallic material having asubstantially higher extrusion temperature than lead to an electriccable body by means of an extrusion press, comprising extruding a lengthof oversize seamless sheath of said metallic material on a length of thecable body during two successive time periods of extrusion separated byan interval for recharging the press, cooling the extruded sheath to asafe temperature, drawing down the cooled sheath to elongate it,augmenting the size of a storage loop of sheathed cable during the firstextrusion period by extruding at a linear speed of extrusion greaterthan the linear speed of drawing, recharging the press during saidinterval and reducing the size of said storage loop during said intervalby drawing down and elongating sheath taken from said storage loopduring said interval, whereby to advance said cable body through thepress during at least part of the said interval.

2. A method of applying a sheath of metallic material having asubstantially higher extrusion temperature than lead to an electriccable body by means of an extrusion press, comprising extruding a lengthof oversize seamless sheath of said metallic material on a length of thecable body during two successive time periods of extrusion separated byan interval for recharging the press, cooling the extruded sheath to asafe temperature, drawing down the cooled sheath to elongate it,augmenting the size of a storage loop of sheathed cable during the firstextrusion period by extruding at a linear speed of extrusion greaterthan the linear speed of drawing, recharging the press during saidinterval and reducing the size of said storage loop during said intervalby drawing down and elongating sheath taken from said storage loopduring said interval, the linear speed of travel of the elongated sheathduring said interval being the same as during first said extrusionperiod, whereby to advance said cable 9 body through the press during atleast'part of the said interval.

3. A method of applying a sheath of metallic material having asubstantially higher extrusion temperature than lead to an electriccable body by means of an extrusion, comprising extruding a length ofoversize seamless sheath of said metallic material on a length of thecable body during two successive time periods of extrusion separated byan interval for recharging the extrusion press, cooling the extrudedsheath to a safe temperature, drawing down the cooled sheath to elongateit, augmenting the size of a storage loop of sheathed cable during thefirst extrusion period by extruding at a linear speed of extrusiongreater than the linear speed of drawing, recharging the press duringsaid interval and reducing the size of said storage loop during saidinterval by drawing down and elongating sheath taken from said storageloop during said interval, the linear speed of travel of the elongatedsheath during said interval being greater than during first saidextrusion period, whereby to advance said cable body through the pressduring at least part of the said interval.

4. A method of applying a sheath of metallic material having asubstantially higher extrusion temperature than lead to an electriccable body by means of an extrusion press, comprising extruding a lengthof oversize seamless sheath of said metallic material on a length of thecable body during two successive time periods of extrusion separated byan interval for recharging the press, cooling the extruded sheath to asafe temperature, passing the cooled sheath through a drawing die toreduce it and elongate it by causing said die during the first extrusionperiod to move in the direction of travel of the sheathed cable body ata speed not exceeding the linear speed of extrusion, recharging thepress during said interval and during said interval causing said die tomove in the reverse direction whereby to draw down and elongate a lengthof extruded sheath and thereby advance said cable body through the pressduring said interval.

5. A method of applying a sheath of metallic material having asubstantially higher extrusion temperature than lead to an electriccable body by means of an extrusion press, comprising extruding a lengthof oversize seamless sheath of said metallic material on a length of thecable body during two successive time periods of extrusion separated byan interval for recharging the press, cooling the extruded sheath to asafe temperature, passing the cooled sheath through a drawing die toreduce it and elongate it by causing said die during the first extrusionperiod to move in the direction of travel of the sheathed cable body ata speed not exceeding the linear speed of extrusion, recharging thepress during said interval and during said interval causing said die tomove in the reverse direction at a linear speed of travel greater thanits linear speed of travel during the first extrusion period, whereby todraw down and elongate a length of extruded sheath and thereby advancesaid cable body through the press during said interval.

6. A method of forming a sheath of metallic material having asubstantially higher extrusion temperature than lead on an electriccable body by means of an extrusion press, comprising extruding anoversize seamless sheath of said metallic material on said cable bodyduring two successive time periods of extrusion separated by an intervalfor recharging the press, cooling said extruded sheath to a safetemperature, storing a length of said cooled extruded sheath during thefirst extrusion period, recharging the press during said interval andoperating upon at least part of said cooled extruded sheath during saidinterval to elongate it and advance said cable body through said press.

7. A method of forming a sheath of metallic material having asubstantially higher extrusion temperature than lead on an electriccable body by means of an extrusion press, comprising extruding anoversize seamless sheath of said metallic material on said cable bodyduring two successive time periods of extrusion separated by an intervalfor recharging the press, cooling said sheath to a safe temperature,storing a length of said cooled extruded sheath during the firstextrusion period, recharging the press during said interval and causingsaid cable body to progress through said stored length of sheath duringsaid interval by operating upon at least part of said stored length ofsheath to elongate it during at least a greater part of said interval.

8. A method of forming a sheath of metallic material having asubstantially higher extrusion temperature than lead on an electriccable body by means of an extrusion press, comprising extruding anoversize seamless sheath of said metallic material on said cable bodyduring two successive time periods of extrusion separated by an intervalfor recharging the press, artificially cooling said sheath as it isformed, storing a length of said cooled extruded sheath during the firstextrusion period, and by operating upon at least part of said cooledextruded sheath during said interval to elongate it, causing anelemental length of said cable body within a hot portion of said sheathto move from said hot portion into a cooled portion of said sheathduring said interval.

9. A method of applying a sheath of metallic material having asubstantially higher extrusion temperature than lead to an electriccable body by means of an extrusion press, comprising extruding anoversize seamless sheath of said metallic material on said cable bodyduring two successive time periods of extrusion separated by an intervalfor recharging the press, cooling said sheath to a safe temperature,recharging the press during said interval and operating upon the cooledsheath to elongate it and advance said cable body through said press atleast during at least the greater part of the said interval.

10. A method of applying a sheath of metallic material having asubstantially higher extrusion temperature than lead to an electriccable body, comprising the successive steps of charging a press withsaid metallic material, extruding said metallic material in the form ofan oversize seamless sheath about a portion of the length of said cablebody, stopping said extrusion to recharge the press, recharging thepress and thereupon resuming the extrusion of said metallic material inthe form of an oversize seamless sheath about a succeeding portion ofthe length of said cable body, cooling said sheath to a safetemperature, and operating upon the cooled sheath during the rechargingof the press to elongate said sheath and advance said cable body throughthe press during at least part of the time occupied by the rechargingoperation.

11. A method of applying a sheath of metallic material having asubstantially higher extrusion temperature than lead to an electriccable body, comprising the successive steps of charging a press withsaid metallic material, extruding said metallic material in the form ofan oversize seamless sheath about a portion of a length of said cablebody, stopping said extrusion to recharge the press, recharging thepress and thereupon resuming the extrusion of said metallic material inthe form of an oversize seamless sheath about a succeeding portion ofthe length of said cable body, artificially cooling said sheath to asafe temperature and operating upon the artificially cooled sheathduring the recharging of the press to elongate said sheath and advancesaid cable body through the press during at least part of the timeoccupied by recharging operation.

12. A method of applying a sheath of metallic material having asubstantially higher extrusion temperature than lead to an electriccable body by means of a billet extrusion press wherein the pressextrudes an oversize seamless sheath of said material on said cable bodyduring a succession of extrusion operations separated by intervals forrecharging the press and wherein the sheath after being cooled to a safetemperature is drawn down to elongate it and reduce the clearancebetween it and said cable body, characterized by the fact that the cablebody is caused to advance through the press during at least the greaterpart of the interval between the extrusion of two successive billets bystoring sheathed cable during an extrusion period preceding saidinterval and drawing down at least part of the stored cable during atleast a greater part of the said interval.

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